* One of the simplest ideas for putting payloads into space is also the
oldest. The idea of blasting an object into orbit goes back to the 17th
century and Isaac Newton's classic treatise on math and physics, PRINCIPIA
MATHEMATICA.

Newton was not serious about space flight. His famous illustration of how a
cannon mounted on top of a mountain could, if given a big enough charge, fire
a cannonball that went clear around the Earth was simply an illustration of
elementary orbital mechanics. However, in the 19th century, French romantic
novelist Jules Verne imagined sending humans to the Moon using a giant
cannon.

Verne's giant cannon was impractical, and illustrated some of the problems
with the idea of simply shooting an object into space with a gun. Unlike a
rocket, an artillery shell fired upward loses energy continuously after
launch, which means that it must have a tremendous muzzle velocity. Since
the length of a "space gun" is necessarily limited, this implies thousands of
gees of acceleration -- and the large muzzle velocity also means that the
projectile will have to endure severe friction and heating effects while
trying to fly up through the dense lower atmosphere.

In any case, 19th-century artillery was too primitive to make the prospect of
putting a payload into orbit a serious proposition. However, development or
large and powerful artillery pieces progressed rapidly after the turn of the
century.

By 1918 Germany had developed an artillery piece of unprecedented range.
This weapon was known as the Wilhelmgeshuetze, or Paris Gun. It had a bore
of 357 millimeters and a barrel length of 30 meters.

The Paris Gun fired a 106 kilogram shell, driven by an explosive charge of
200 kilograms that produced an acceleration of 7,500 gees and a muzzle
velocity of almost 6,000 kilometers per hour. The gun's maximum range was
126 kilometers, with the shell reaching a peak altitude of almost 42
kilometers during its three minutes of flight.

While similar large long-range artillery pieces were used as late as World
War II, the development of aircraft and rockets provided a much more
effective way to deliver munitions over long distances, and the development
of bigger and more powerful artillery pieces ended.

Use of such large guns for space launch remained a possibility, however. The
maximum possible muzzle velocity of an artillery piece charged with
nitrocellulose explosives is sufficient to launch small probes to high
altitude for atmospheric sounding applications, and in the mid-1960s
experiments along this line were performed using lengthened US Navy surplus
406 millimeter (16 inch) guns.

The effort was designation HARP, for High Altitude Research Project, and was
the brainchild of Gerald Bull and a group at McGill University in Canada --
with support by Charles Murphy of the US Army Research Office and Aberdeen
Proving Ground.

Bull's group devised a fin-stabilized projectile named Martlet for cannon
launch. As the Martlet had a much smaller diameter than the cannon bore, it
was fired using a snug-fitting "sabot", or shoe, that was discarded after the
Martlet left the muzzle.

About 200 Martlet 2s were launched with the 406 millimeter guns, with most of
the launches from the island of Barbados in the Carribbean but a few from
Yuma Proving Ground in Arizona. The Martlet 2s carried various payloads,
including chemical releases and ruggedized instruments. They were fired to
altitudes of up to 180 kilometers.

Smaller projectiles were launched from 127 millimeter and 178 millimeter (5
and 7 inch) guns to altitudes of about 75 kilometers from Yuma and the US
National Aeronautics & Space Administration's (NASA's) launch facility at
Wallops Island, Virginia. A total of about 570 ballistic projectiles were
launched in the course of HARP.

While HARP blasted projectiles into space, the McGill group was driving the
development of cannon-launched rockets to put payloads into orbit. Their
Martlet 3 design was a discarding-sabot solid-propellant rocket with a
diameter of 190 millimeters (7.5 inches), and was to be launched from a 406
millimeter gun.

The Martlet 3 was to lead to the Martlet 4, which was to be a multistage
cannon-launched rocket with a launch mass of 1.2 tonnes and a payload
capacity of 90 kilograms to low Earth orbit (LEO); it would be given a muzzle
velocity of 5,400 KPH. The McGill group also considered a three-stage rocket
design that could put 295 kilograms into a 185 kilometer orbit using all
solid fuel, or 590 kilograms into a 1,100 kilometer orbit using all liquid
fuel. This vehicle would be launched from a 813 millimeter (32 inch) gun.

Development of these cannon-launched projectiles proceeded to the point where
subsystems were test-launched, demonstrating survival under accelerations of
up to 10,000 gees. Subsystems included solid-rocket motors, an IR horizon
sensor, a spin-rate sensor, Sun sensors, NiCad batteries, a solenoid-operated
cold gas thruster, and various support electronics modules.

The McGill group eventually concentrated on a rocket-propelled variant of the
Martlet 2, named the Martlet 2G-1, as a minimum alternative to the ambitious
Martlet 4. The Martlet 2G-1 would have been able to put a two kilogram
payload into LEO, making it an excellent demonstrator for a cannon-based
"nanosatellite" launch system.

Unfortunately, funding for HARP eventually dried up and disappeared, even
though the Martlet 2G-1 and various Martlet 3 rockets had been designed and
were under construction.

Although HARP was discontinued, it was the most impressive effort ever made
to blast payloads into space using a cannon -- and in fact appears to be the
only project that ever succeeded in doing so. It was also groundbreaking in
developing rocket technology for launch by artillery, and in developing
instrument and guidance systems that could withstand the stresses of being
fired out of a gun.

Eventually, guided munitions that could be fired out of cannon, such as the
American Copperhead laser-seeking 155 millimeter round, were developed and
deployed, but Bull's dream of using a cannon to put a payload into orbit
remains unrealized.

* The story of Gerald Bull didn't end with HARP, however, and took a turn
straight out of James Bond (and in fact was dramatized in a movie made for US
TV). Bull was embittered by the termination of HARP, and in 1980 served a
short term in a US prison as part of a plea bargain for charges of smuggling
arms to South Africa.

After he was released, Bull was unable to interest anyone in the US in his
superguns, and so moved to Brussels and peddled his designs to anyone who
would pay -- first to the Chinese, then to the Iraqis. This was a fatal
mistake in judgement; Bull was gunned down in front of his home in early
1990, apparently by agents of the Israeli Mossad intelligence agency.

Three weeks after Bull's death, British customs seized components of an
extremely large-caliber gun that were being readied for shipment to Iraq,
disguised as pipe sections. After the Iraqi defeat in the Gulf War in 1991,
UN inspectors operating in Iraq discovered an incomplete 350 millimeter
supergun with a fixed elevation, and parts for an even bigger 1,000
millimeter supergun.

* The muzzle velocities that can be obtained with a cannon driven by
nitrocellulose explosives are limited, and so research has been conducted on
alternatives.

One such alternative is the light gas gun, which was invented in the postwar
period as a means of performing hypersonic experiments with missile warhead
reentry vehicle designs, and studying the risks of space debris to
spacecraft.

Obtaining high velocities in a cannon requires a gas with a high speed of
sound, exerting high pressures on the base of a projectile through a long
barrel. The speed of sound squared varies inversely with the molecular
weight of the gas and directly with the gas temperature, meaning that a hot
gas of low molecular weight makes an excellent propellant for a space gun.

A light gas gun uses a piston to rapidly compress a reservoir of helium gas.
This reservoir is sealed off from the gun barrel by a diaphragm; when the
diaphragm breaks, the hot gas expands rapidly and blasts a projectile down
the barrel. Light gas guns using hydrogen instead of helium are expected to
have even better performance.

There have been several research programs conducted on light gas guns. One
of the most significant was led by John Hunter of the US Lawrence Livermore
National Laboratory. Hunter is now promoting a commercial scheme for a light
gas gun, appropriately named the Jules Verne Launcher, for delivering small
payloads to orbit.

* Electromagnetic guns have been one of the most prominent alternative
technologies for space cannon. Research has been conducted on two different
approaches: railguns and coilguns.

A railgun consists of a pair of copper rails, mounted in an insulating
barrel, with the rails connected to a rapidly switched high current source.
An armature on the projectile to be fired completes the circuit, resulting in
a magnetic force that drives the projectile down the barrel. This armature
is usually actually a plasma arc ignited at the base of the projectile.

Switching such high currents has proven tricky in practice. Railguns also
suffer from erosion of the rails after a few launches, and the designs based
on plasma arcs have difficulties with uncontrolled arcing around the
projectile or to the muzzle. Railgun enthusiasts have proposed designs that
they claim will be able to boost a ten kilogram projectile to 36,000 KPH, but
so far railguns have been restricted to lab-scale systems with muzzle
velocities no greater than 21,600 KPH.

Coilguns are a little more intuitive in design. They consist of a series of
pulsed electromagnetic coils that accelerate a projectile to high velocity.
They are more mechanically complicated than railguns, but since there is no
direct contact between the projectile and the coils they avoid the erosion
and arc-over problems of railguns.

"Mass drivers" based on coilguns were considered for launching payloads from
the Moon at least as far back as the 1960s, and small-scale models have been
built for decades. NASA has designed a coilgun that can accelerate 10
kilograms to 39,600 KPH; an enhanced version of this device has been proposed
to boost a 300 kilogram rocket to 36,000 KPH, allowing it to put a 150
kilogram payload into LEO.

However, so far coilguns have lagged railguns in performance. A major
drawback to both railguns and coilguns is that any facility using them would
be big and very expensive.

* A new alternative for a space gun, the ram accelerator, has been promoted
by Abraham Hertzberg and colleagues at the University of Washington since
1988.

The ram accelerator consists of a long, sealed tube filled with a mixture of
fuel and oxidizer, such as hydrogen and oxygen. A projectile resembling the
centerbody of a ramjet is shot into the tube at a velocity of about 3,600
KPH, igniting the mixture and blasting the projectile down the tube, which
acts like the outer cowling of a ramjet.

It is possible to accelerate the projectile in several distinct modes by
varying the fuel-oxidizer mix in different sections of the launch tube, with
the sections isolated by thin diaphragms that are ruptured by the projectile
as it speeds up the tube.

While there have been proposals to build ram accelerators to launch one-tonne
projectiles for delivering supplies to LEO, so far these devices have
remained lab experiments. The University of Washington group is currently
operating a three stage, 120 millimeter ram accelerator that launches 4.3
kilogram projectiles with a muzzle velocity of 4,320 KPH.

* All the options for space cannon face the same constraints that Jules
Verne's Moon gun would have had to contend with.

The fact that a projectile leaving the muzzle of the space cannon loses
energy from that instant on means that it has its highest velocity during the
part of its flight path that moves through the densest parts of the
atmosphere.

As a result, the projectile must be able to withstand frictional heating and
must also be given additional muzzle velocity to overcome the losses it will
suffer. A simple calculation based on a 1-kilogram cubic projectile launched
at a muzzle velocity of 39,600 KPH at sea level shows that it will lose 20%
of its velocity and a good part of its ablative thermal protection in the
first 16 meters of flight.

One way of minimizing these losses is to launch the projectiles from the top
of a mountain. Calculations show that launch energy requirements are cut by
almost a third if the cannon's muzzle is placed on a mountaintop at an
altitude of 4.6 kilometers (15,000 feet).

The energy requirements for launching large payloads with a space gun are
still extreme, however, and so the approach appears best suited to launch of
large numbers of small "hardened" payloads. Constellations of
"nanosatellites" for communications or similar applications could be placed
in orbit at relatively low cost, using the space gun as a "first stage" for
launch of a rocket-boosted projectile. Such a projectile would weigh about a
tonne and carry about a 60 kilogram payload; the space gun would have to
accelerate it to a muzzle velocity of 9,000 to 14,400 KPH.

None of the space gun technologies investigated to date have been scaled up
to this size, and doing so would require a major capital investment that
would demand high launch rates for a long period of time to break even.
However, building such a gun does not require the development of any major
new technologies and remains an interesting possibility for the future.